Among the transmission/reception switch circuits for application to ultrasonographs, there have been switch circuits each of which, having two MOS transistors, a main switch connected to a common source of the two MOS transistors, a drive circuit connected to a common gate of the two MOS transistors and a holding circuit, holds an on-state or off-state of the switch (see Japanese Unexamined Patent Application Publication No. 2004-363997, for example).